Recently in the field of semi-conductors, a lead titanate zirconate (PZT) film, a barium strontium titanate (BST) film, a tantalic acid strontium bismuth (SBT) film, a titanic acid zirconate lantern lead (PLZT) film, etc., each having a high dielectric constant and also a high step coverage has been used as an oxide-based dielectric film for a semiconductor memory. As a manufacturing method of the dielectric film, feeding organic metal ingredient gas and oxidized gas to the semi-conductor manufacturing equipment holding a substrate and chemically vapor depositing is generally known. For example, Pb(DPM)2 or so as Pb source, Zr(DPM)4 or so as Zr source, and Ti(OiPr)2(DPM)2 or so as Ti source are employed as the organic metal ingredient material each being dissolved in organic solvent and made gaseous by a vaporizer, together with employing oxygen, nitrogen dioxide and the like as the oxidized gas in the case where producing PZT film.
In the case where nitrogen dioxide is employed as oxidative gas in the semi-conductor manufacturing, nitrogen oxides and organic solvent are discharged as the exhaust gas along with a slight amount of organometallic compound. Among these components in the exhaust gas, organometallic compounds are easily removed by means of a dry cleaning process at an ordinary temperature or a cold trap and the like because the compounds are contained in a slight amount. On the other hand, because the concentration of nitrogen oxides and organic solvent in the exhaust gas is usually large in the order of several thousands to several tens of thousands ppm extravagantly exceeding environmental criteria and extremely gives adverse effects to human organism and environment, it is necessary to clean the exhaust gas including these prior to discharging the gas into the atmosphere.
Conventionally as the cleaning process of exhaust gas containing nitrogen oxides, there have been a wet process, an adsorption process, a non-catalytic reduction process, a catalytic reduction process, etc. Among these cleaning processes, the wet process cleans the exhaust gas containing nitrogen oxides by making it being absorbed by aqueous alkali absorber directly or after making it easily being absorbable by the aqueous alkali absorber as a result of converting nitrogen oxides in the exhaust gas into nitrogen dioxide with a catalyst. The adsorption process cleans the exhaust gas containing nitrogen oxides by physically or chemically adsorbing nitrogen oxides to an adsorbing agent such as activated carbon, zeolite, etc. The non-catalytic reduction process generally cleans the exhaust gas containing nitrogen oxides by adding reductive gas such as ammonia or the like, and by reductively decomposing nitrogen oxides into nitrogen and water while heating them. The catalytic reduction process is currently popular and generally cleans the exhaust gas containing nitrogen oxides by adding the reductive gas such as ammonia, hydrocarbon or the like, and bringing them into contact with a catalyst comprising metal or metal compound while heating them resulting in reductively decomposing nitrogen oxides into nitrogen and water.
As a cleaning process of exhaust gas containing organic solvent, there is a combustion cleaning process which introduces the exhaust gas containing organic solvent into a flame of combustible gas such as propane or the like and oxygen or air and burns them, or a catalytic cleaning process which adds oxygen or air to the exhaust gas containing organic solvent and then oxidatively decomposes them by bringing them into contact while heating with a catalyst supporting noble metal or metal oxide on an inorganic carrier.
However, the foregoing wet process practically requires converting nitrogen oxide into nitrogen dioxide with the use of catalyst because nitrogen oxide is not easily absorbed by aqueous alkali absorber while nitrogen dioxide is easily absorbed. Accordingly, the wet process had shortcomings of needing large-scale apparatus and hazardous post processing of the absorber after the usage.
Moreover, the foregoing adsorption process had problems that the cleaning capability (throughput of nitrogen oxides per unit amount of an adsorbing agent) was small and that there was an anxiety of adsorbed nitrogen oxides' desorption during adsorption operation depending on the operative condition.
Further, the foregoing non-catalytic reduction process was not suitable for cleaning the exhaust gas containing nitrogen oxides with high concentration discharged from the manufacturing process of semiconductor not only because it was necessary to elevate the temperature of treating the exhaust gas as high as close to 1,000° C., but also because the decomposition factor of reductively decomposing nitrogen oxides into nitrogen and water was around 50 to 60%.
The foregoing catalytic reduction process is a superior cleaning process capable of reductively decomposing nitrogen oxides at relatively low temperature further achieving decomposition factor of 90% or greater. However, in the case where the amount of the reductive gas added in the occasion of cleaning the exhaust gas containing nitrogen oxides of high concentration is small, the decomposition of nitrogen oxides becomes not enough thereby allowing discharge of nitrogen oxides exceeding acceptable concentration. On the contrary, in the case where the amount of the above reductive gas is large, harmful gases such as carbon monoxide, hydrocarbon gas and so on are discharged. Accordingly, it was difficult controlling not to discharge harmful gas in accordance with the foregoing catalytic reduction process applying to cleaning the exhaust gas containing nitrogen oxides with varying condition such as kinds, concentration and the like discharged from the manufacturing process of semiconductor. Furthermore, in the case where organic solvent was contained along with nitrogen oxides in the exhaust gas, it was more difficult controlling to prevent discharging harmful gases.